For dining rooms it is best to always leave enough space on either side of the table for chairs being pulled in or out. Leave enough space so that people will not ‘catch’ the back of the rug with their chair. Your rug needs to be approximately 1.2m - 1.5m wider and longer than your table (i.e 60-75cm around the perimeter of the table for chairs).
When furniture such as couches and sofas are central in an open plan living room, your rug should be big enough to fit under all four legs of each piece of furniture. This will define the space and be proportionate to the size of furniture and space surrounding.
In smaller rooms where your furniture is up against the wall, the rug should be large enough to fit under the front legs of all pieces.